Lecture 1:Object Oriented Programming pdf

Object-Oriented Programming  The unit in procedural programming is function, and unit in object-oriented programming is class  Procedural programming concentrates on creating functi

Lecture 1:

Object Oriented Programming

Procedural vs Object-Oriented

Programming

 The unit in procedural programming is function, and unit

in object-oriented programming is class

 Procedural programming concentrates on creating

functions, while object-oriented programming starts from isolating the classes, and then look for the methods inside them

 Procedural programming separates the data of the

program from the operations that manipulate the data,

while object-oriented programming focus on both of them

figure1: procedural figure2: object-oriented

Concept of Class and Object

 “Class” refers to a blueprint It defines the

variables and methods the objects support

 “Object” is an instance of a class Each object has a class which defines its data and behavior

Class Members

 A class can have three kinds of members:

 fields: data variables which determine the status

of the class or an object

 methods: executable code of the class built from

statements It allows us to manipulate/change the status of an object or access the value of the data member

 nested classes and nested interfaces

Sample class

class Pencil {

public String color = “red”;

public int length;

public float diameter;

public static long nextID = 0;

public void setColor (String newColor) { color = newColor;

}

Fields – Declaration

 Field Declaration

 a type name followed by the field name, and optionall

y an initialization clause

 primitive data type vs Object reference

 boolean, char, byte, short, int, long, float, double

 field declarations can be preceded by different modifi ers

 access control modifiers

 static

 final

More about field modifiers (1)

 Access control modifiers

 private : private members are accessible only in the

class itself

 package : package members are accessible in

classes in the same package and the class itself

 protected : protected members are accessible in

classes in the same package, in subclasses of the

class, and in the class itself

 public : public members are accessible anywhere the

class is accessible

public class Pencil {

public String color = “red”;

public int length;

public float diameter;

private float price;

public static long nextID = 0;

public void setPrice (float newPrice) { price = newPrice;

} }

public class CreatePencil {

public static void main (String args[]){

Pencil p1 = new Pencil();

p1.price = 0.5f;

}

}

Pencil.java

CreatePencil.java

%> javac Pencil.java

%> javac CreatePencil.java

CreatePencil.java:4: price has private access in Pencil

p1.price = 0.5f;

^

More about field modifiers (2)

 static

 only one copy of the static field exists, shared by all o bjects of this class

 can be accessed directly in the class itself

 access from outside the class must be preceded by th

e class name as follows

System.out.println(Pencil.nextID);

or via an object belonging to the class

from outside the class, non-static fields must be acces

public class CreatePencil {

public static void main (String args[]){

Pencil p1 = new Pencil();

Pencil.nextID++;

System.out.println(p1.nextID);

//Result?

Pencil p2 = new Pencil();

Pencil.nextID++;

System.out.println(p2.nextID);

//Result?

System.out.println(p1.nextID);

//Result?

}

}

1

still 2!

2

Note: this code is only for the purpose of showing the usage of static fields It has POOR design!

More about field modifiers (3)

 final

 once initialized, the value cannot be changed

 often be used to define named constants

 static final fields must be initialized when the class is ini tialized

 non-static final fields must be initialized when an object

of the class is constructed

Fields –Initialization

 Field initialization

 not necessary to be constants, as long as with the rig

ht type

 If no initialization, then a default initial value is assigne

d depending on its type

Type Initial Value

byte, short, int, long 0

object reference null

Methods – Declaration

 Method declaration: two parts

1. method header

 consists of modifiers (optional), return type, method name,

parameter list and a throws clause (optional)

 types of modifiers

• access control modifiers

• abstract

 the method body is empty E.g.

abstract void sampleMethod( );

• static

 represent the whole class, no a specific object

 can only access static fields and other static methods of the sam

e class

Methods – Invocation

 Method invocations

 invoked as operations on objects/classes using the d

ot ( ) operator

reference.method(arguments)

 static method:

 Outside of the class: “reference” can either be the class na

me or an object reference belonging to the class

 Inside the class: “reference” can be ommitted

 non-static method:

 “reference” must be an object reference

Method - Overloading

 A class can have more than one method with the same n ame as long as they have different parameter list

public class Pencil {

.

public void setPrice (float newPrice) {

price = newPrice;

}

public void setPrice (Pencil p) {

price = p.getPrice();

} }

 How does the compiler know which method you’re invoki

Methods – Parameter Values

 Parameters are always passed by value

public void method1 (int a) {

a = 6;

}

public void method2 ( ) {

int b = 3;

method1(b); // now b = ?

// b = 3

}

 When the parameter is an object reference, it is the objec

t reference, not the object itself, getting passed

 Haven’t you said it’s past by value, not reference ?

class PassRef{

public static void main(String[] args) {

Pencil plainPencil = new Pencil("PLAIN");

System.out.println("original color: " +

plainPencil.color);

paintRed(plainPencil);

System.out.println("new color: " +

plainPencil.color);

}

public static void paintRed(Pencil p) {

p.color = "RED";

p = null;

}

}

another example: (parameter is an object reference)

plainPencil

plainPencil

plainPencil p

plainPencil p

color: PLAIN

- If you change any field of the object which the parameter refers to, the object is changed for every variable which holds a reference to this object

color: PLAIN

color: RED

color: RED NULL

p

The Main Method - Concept

 main method

 the system locates and runs the main method for a class when you run a program

 other methods get execution when called by the main method explicitly or implicitly

 must be public, static and void

The Main Method - Getting Input from the Command Line

 When running a program through the java command, you can provi

de a list of strings as the real arguments for the main method In the main method, you can use args[index] to fetch the correspondin

g argument

class Greetings {

public static void main (String args[]){

String name1 = args[0];

String name2 = args[1];

System.out.println("Hello " + name1 + “&“ +name2); }

}

 java Greetings Jacky Mary

Hello Jacky & Mary

Modifiers of the classes

 A class can also has modifiers

 public

 publicly accessible

 without this modifier, a class is only accessible within its own package

 abstract

 no objects of abstract classes can be created

 all of its abstract methods must be implemented by its subclass; other wise that subclass must be declared abstract also

 final

 can not be subclassed

 Normally, a file can contain multiple classes, but only one public one The file name and the public class name should be the sam e